1. Field of the Invention
This invention relates to a composite hollow fiber, particularly to a composite hollow fiber which is excellent in permeability and separation performance of gases and which has good heat resistance, humidity resistance and mechanical strength.
2. Description of the Prior Art
Gas separation membranes are required first to have a good gas permeation rate, secondly a good separation factor for the gas component to be separated relative to the remaining gas components of the gaseous mixture, and thirdly heat resistance and humidity resistance to be able to withstand the temperature and humidity to which the membrane may be subjected during the gas separation operation, and the mechanical strength to withstand the pressure stresses and the like.
For increasing the gas permeation rate [cm.sup.3 (STP)/cm.sup.2.sec.cmHg] with the use of a material having a small gas permeation coefficient [cm.sup.3 (SIP)cm/cm.sup.2.sec.cmHg], it is required to make the membrane thinner or to enlarge the permeation surface area, and for this purpose, studies have been made about various thin membrane techniques or hollow fiber techniques. However, no satisfactory gas separation membrane has yet been obtained.
As a polymer having a large gas permeation coefficient, polysiloxane has been known but the membrane of this material has low mechanical strength. For improvement of the mechanical strength of the polysiloxane membrane, a gas separation membrane composed of a polysiloxanepolycarbonate copolymer has been proposed (U.S. Pat. No. 3,980,456), but this membrane has generally small gas separation factors and also insufficient mechanical strength.
For increasing the permeation surface area and the mechanical strength of the gas separation membrane, composite membranes having coatings of polysiloxane or siloxane copolymer on porous hollow fiber substrates have been proposed (Japanese Laid-open patent Publication No. 86684/1978). However, in practice, they have an unsatisfactory gas permeation rate and insufficient resistance to heat and humidity and mechanical strength of the coating layer; therefore, there is involved the disadvantage that the coating layer tends to readily peel off.